1COMPRESS(1) General Commands Manual COMPRESS(1)
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6 compress, uncompress, zcat - compress and expand data
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9 compress [ -f ] [ -k ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ -- ] [ name
10 ... ]
11 uncompress [ -f ] [ -k ] [ -v ] [ -c ] [ -V ] [ -- ] [ name ... ]
12 zcat [ -V ] [ -- ] [ name ... ]
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15 Compress reduces the size of the named files using adaptive Lempel-Ziv
16 coding. Whenever possible, each file is replaced by one with the ex‐
17 tension .Z, while keeping the same ownership modes, access and modifi‐
18 cation times. If no files are specified, the standard input is com‐
19 pressed to the standard output. Compress will only attempt to compress
20 regular files. In particular, it will ignore symbolic links. If a file
21 has multiple hard links, compress will refuse to compress it unless the
22 -f flag is given.
23 If -f is not given and compress is run in the foreground, the user is
25 prompted as to whether an existing file should be overwritten.
26 Compressed files can be restored to their original form using uncom‐
28 press or zcat.
29 uncompress takes a list of files on its command line and replaces each
31 file whose name ends with .Z and which begins with the correct magic
32 number with an uncompressed file without the .Z. The uncompressed file
33 will have the mode, ownership and timestamps of the compressed file.
34 The -k option makes compress/uncompress keep the input files instead of
36 automatically removing them.
37 The -c option makes compress/uncompress write to the standard output;
39 no files are changed.
40 zcat is identical to uncompress -c. zcat uncompresses either a list of
42 files on the command line or its standard input and writes the uncom‐
43 pressed data on standard output. zcat will uncompress files that have
44 the correct magic number whether they have a .Z suffix or not.
45 If the -r flag is specified, compress will operate recursively. If any
47 of the file names specified on the command line are directories, com‐
48 press will descend into the directory and compress all the files it
49 finds there. When compressing, any files already compressed will be
50 ignored, and when decompressing, any files already decompressed will be
51 ignored.
52 The -V flag tells each of these programs to print its version and
54 patchlevel, along with any preprocessor flags specified during compila‐
55 tion, on stderr before doing any compression or uncompression.
56 Compress uses the modified Lempel-Ziv algorithm popularized in "A Tech‐
58 nique for High Performance Data Compression", Terry A. Welch, IEEE Com‐
59 puter, vol. 17, no. 6 (June 1984), pp. 8-19. Common substrings in the
60 file are first replaced by 9-bit codes 257 and up. When code 512 is
61 reached, the algorithm switches to 10-bit codes and continues to use
62 more bits until the limit specified by the -b flag is reached (default
63 16). Bits must be between 9 and 16. The default can be changed in the
64 source to allow compress to be run on a smaller machine.
65 After the bits limit is attained, compress periodically checks the com‐
67 pression ratio. If it is increasing, compress continues to use the ex‐
68 isting code dictionary. However, if the compression ratio decreases,
69 compress discards the table of substrings and rebuilds it from scratch.
70 This allows the algorithm to adapt to the next "block" of the file.
71 Note that the -b flag is omitted for uncompress, since the bits parame‐
73 ter specified during compression is encoded within the output, along
74 with a magic number to ensure that neither decompression of random data
75 nor recompression of compressed data is attempted.
76 The amount of compression obtained depends on the size of the input,
78 the number of bits per code, and the distribution of common substrings.
79 Typically, text such as source code or English is reduced by 50-60%.
80 Compression is generally much better than that achieved by Huffman cod‐
81 ing (as used in pack), or adaptive Huffman coding (compact), and takes
82 less time to compute.
83 Under the -v option, a message is printed yielding the percentage of
85 reduction for each file compressed.
86 -- may be used to halt option parsing and force all remaining arguments
88 to be treated as paths.
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91 Exit status is normally 0; if the last file is larger after (attempted)
92 compression, the status is 2; if an error occurs, exit status is 1.
93 Usage: compress [-dfvcVr] [-b maxbits] [file ...]
95 Invalid options were specified on the command line.
96 Missing maxbits
97 Maxbits must follow -b.
98 file: not in compressed format
99 The file specified to uncompress has not been compressed.
100 file: compressed with xx bits, can only handle yy bits
101 File was compressed by a program that could deal with more bits
102 than the compress code on this machine. Recompress the file
103 with smaller bits.
104 file: already has .Z suffix -- no change
105 The file is assumed to be already compressed. Rename the file
106 and try again.
107 file: filename too long to tack on .Z
108 The file cannot be compressed because its name is longer than
109 12 characters. Rename and try again. This message does not
110 occur on BSD systems.
111 file already exists; do you wish to overwrite (y or n)?
112 Respond "y" if you want the output file to be replaced; "n" if
113 not.
114 uncompress: corrupt input
115 A SIGSEGV violation was detected which usually means that the
116 input file has been corrupted.
117 Compression: xx.xx%
118 Percentage of the input saved by compression. (Relevant only
119 for -v.)
120 -- not a regular file or directory: ignored
121 When the input file is not a regular file or directory, (e.g. a
122 symbolic link, socket, FIFO, device file), it is left unal‐
123 tered.
124 -- has xx other links: unchanged
125 The input file has links; it is left unchanged. See ln(1) for
126 more information. Use the -f flag to force compression of mul‐
127 tiply-linked files.
128 -- file unchanged
129 No savings is achieved by compression. The input remains vir‐
130 gin.
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133 Although compressed files are compatible between machines with large
134 memory, -b12 should be used for file transfer to architectures with a
135 small process data space (64KB or less, as exhibited by the DEC PDP se‐
136 ries, the Intel 80286, etc.)
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139 pack(1), compact(1)
140 local COMPRESS(1)